pubs.acs.org/Langmuir © 2009 American Chemical Society
Mechanistic Study on Demulsification of Water-in-Diluted Bitumen Emulsions by Ethylcellulose Xianhua Feng, Paolo Mussone, Song Gao, Shengqun Wang, Shiau-Yin Wu, Jacob H. Masliyah, and Zhenghe Xu* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada Received August 10, 2009. Revised Manuscript Received September 21, 2009 In our previous study, ethylcellulose (EC), an effective, nontoxic, and biodegradable natural polymer, was found effective in dewatering water-in-diluted bitumen emulsions. In this study, the demulsification mechanism of water-indiluted bitumen emulsions by EC is investigated. In situ experiments using a micropipet apparatus provided direct evidence on both flocculation and coalescence of water droplets in diluted bitumen by EC. The addition of EC was found to decrease naphtha-diluted bitumen-water interfacial tension significantly. At the molecular level, AFM imaging revealed disruption of the continuous interfacial films formed from surface-active components of bitumen by EC. Our study clearly indicates that the demulsification by EC is through both flocculation and coalescence of water droplets, attained by competitive adsorption of EC at the oil-water interface and disruption of the original protective interfacial films formed from the surface-active components of bitumen.
Introduction Emulsification and emulsion stability play a critical role in many industrial practices such as food processing,1,2 pharmaceutical formulation,3,4 manufacture of cosmetics,5 emulsion adhesives,6 emulsion coatings,7 petroleum processing,8 emulsion polymerization, and more recently nanosynthesis.9,10 As a system of liquid droplets dispersed in another immiscible liquid, emulsions are thermodynamically unstable due to surface energy of liquid-liquid interfaces. In most cases, surface-active agents, i.e., surfactants, are used to ensure emulsion stability by reducing the interfacial energy and creating steric and/or hydration repulsion between the dispersed droplets. On the other hand, stable emulsions are undesirable in petroleum processing because they cause high transportation cost and damage to processing facilities.8 Although great effort has been made to destabilize these emulsions by chemical additives, it remains a challenge to effectively break up undesired emulsions due to lack of in-depth understanding on molecular mechanisms of chemical demulsification. In order to better control emulsion stability and achieve effective demulsification, it is important to understand the mechanism of chemical demulsification. Understanding demulsification also provides implications for improving emulsification as encountered in many industries and improves our understanding of
interfacial phenomena. In this study, we investigate molecular mechanism of demulsifying a water-in-diluted bitumen emulsion by a biodegradable, natural polymer. The methodology and fundamental knowledge derived from this study are equally applicable to other important chemical demulsification systems. Water-in-oil (W/O) emulsions are often encountered during the production of crude oils and bitumen. In crude oil recovery, W/O emulsions are formed when crude oil and water are intimately mixed at the wellhead, production well, valves, and pumps.8 Similarly, in bitumen recovery from mined oil sands using warm water, followed by dilution of bitumen froth with naphtha and a two-stage centrifugation, 2-4 wt% water remains in the final diluted bitumen product in the form of stable emulsified water droplets of several micrometers in size.11 The presence of surface-active chemical species in crude oil and bitumen, such as asphaltenes, resins, and naphthenic acids as well as fine solid particles at the interface between water droplets and continuous organic phase has been reported extensively.8,12-16 These adsorbed materials form a rigid interfacial film as a barrier preventing water droplets from attaching to each other and coalescing. Recent studies revealed that only a small fraction of asphaltenes, a constituent of bitumen, participates in the formation of interfacial films.17,18 By definition, asphaltenes are a solubility class of petroleum materials that are soluble in toluene and insoluble in n-alkanes.
*To whom correspondence should be addressed: Tel 1-780-492-7667; Fax 1-780-492-2881; e-mail
[email protected]. (1) Dalgleish, D. G. Food Hydrocolloids 2006, 20, 415–422. (2) Jung, S.; Maurer, D.; Johnson, L. A. Bioresour. Technol. 2009, 100, 5340– 5347. (3) Tadros, T. F. Pestic. Sci. 1989, 26, 51–77. (4) Tadros, T. F. Adv. Colloid Interface Sci. 1993, 46, 1–47. (5) Gutierrez, J. M.; Gonzalez, C.; Maestro, A.; Sole, I.; Pey, C. M.; Nolla, J. Curr. Opin. Colloid Interface Sci. 2008, 13, 245–251. (6) Jovanovic, R.; Dube, M. A. J. Macromol. Sci., Polym. Rev. 2004, C44, 1–51. (7) Swarup, S.; Schoff, C. K. Prog. Org. Coat. 1993, 23, 1–22. (8) Angle, C. W. Chemical demulsification of stable crude oil and bitumen emulsions in petroleum recovery - a review. In Encyclopedic Handbook of Emulsion Technology; Sjoblom, J., Ed.; Marcel Dekker: New York, 2001; pp 541-594. (9) Holmberg, K. Eur. J. Org. Chem. 2007, 731–742. (10) Castelvetro, V.; De Vita, C. In Nanostructured Hybrid Materials from Aqueous Polymer Dispersions ; Elsevier Science: Amsterdam, 2004; pp 167-185.
3050 DOI: 10.1021/la9029563
(11) Long, Y. C.; Kan, J.; Walker, A.; Dabros, T. Process for treating heavy oil emulsions using a light aliphatic solvent-naphtha mixture. Canada Patent 2435113, 2005. (12) Mullins, O. C.; Sheu, E. Y. Structures and Dynamics of Asphaltenes; Plenum Press: New York, 1998. (13) Gu, G.; Xu, Z.; Nandakumar, K.; Masliyah, J. H. Fuel 2002, 81, 1859–1869. (14) Yan, Z. L.; Elliott, J. A. W.; Masliyah, J. H. J. Colloid Interface Sci. 1999, 220, 329–337. (15) Gafonova, O. V.; Yarranton, H. W. J. Colloid Interface Sci. 2001, 241, 469– 478. (16) Yang, X. L.; Verruto, V. J.; Kilpatrick, P. K. Energy Fuels 2007, 21, 1343– 1349. (17) Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Czarnecki, J.; Wu, X. A. Energy Fuels 2007, 21, 963–972. (18) Czarnecki, J. Energy Fuels 2009, 23, 1253–1257.
Published on Web 10/09/2009
Langmuir 2010, 26(5), 3050–3057
Feng et al.
Extensive chemical analysis revealed that asphaltene molecules have remarkably complex molecular structures building on fused aromatic rings with saturated substituents12,19,20 and polar groups such as amine, hydroxyl, carboxyl, and sulfur-containing functional groups, making them amphiphilic.12,17,21 Masliyah et al.22 reported that asphaltenes adsorbed at an oil-water interface had a lower H/C (1.1) ratio and a higher O/C (0.03) ratio, as compared with H/C of 1.2-1.3 and O/C of 0.01-0.02 for those solubilized in the oil phase, showing a direct correlation between the amount of oxygen atoms at the oil-water interface and emulsion stability. Moreover, asphaltene molecules can associate to form a crosslinked, three-dimensional network at the oil-water interface, which makes emulsions extremely stable.23 To achieve efficient breakup of natural W/O emulsions, demulsifiers must have a stronger affinity for the interface than the native emulsion stabilizers and the ability to weaken the oilwater interfacial film. Effective demulsifiers often lower oilwater interfacial tension and accelerate intervening liquid film thinning rate, thereby promoting coalescence of water droplets.24-26 Zhang et al.27 reported that in the presence of a commercial ethylene oxide (EO)/propylene oxide (PO) copolymer demulsifier the asphaltene film at oil-water interface became significantly less rigid and the asphaltene aggregates at the interface became less packed. Ese et al.28 studied the influence of some commercial complex block copolymer demulsifiers on the properties of asphaltene films at an oil-water interface using a Langmuir balance. They concluded that higher molecular weight commercial demulsifiers could reduce the rigidity of asphaltene films at oilwater interface. At optimal dosages, the demulsifiers could prevent asphaltenes from forming interfacial films at the oil-water interface. Recently, Daniel-David et al.29 found that poly(alkylene oxide)/poly(dimethysiloxane)/poly(alkylene oxide) triblock copolymers are able to disrupt the network of asphaltene aggregates at an oil-water interface and induce breakup of emulsions. The demulsification process has been correlated to the molecular structure of demulsifiers. For example, Pena et al.30 found that alkylphenolformaldehyde resins modified with EO/PO promoted fast coalescence of water droplets, whereas polyurethane resins modified with EO/PO promoted fast flocculation with slow coalescence. A combination of these two demulsifiers resulted in a much faster separation of water from W/O emulsions because of synergistic action. A recent report by Zhang et al. demonstrated that a combination of flocculation and coalescence of water droplets by demulsifiers with a highly branched structure such as diethylene(19) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677–684. (20) Mullins, O. C. SPE J. 2008, 13, 48–57. (21) Mitrakirtley, S.; Mullins, O. C.; Vanelp, J.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252–258. (22) Masliyah, J.; Xu, Z.; Nandakumar, K.; Yeung, A.; Czarnecki, J. In Emulsions studies associated with bitumen recovery from Canadian oil sands: Part I.; Proceedings of the 3rd International Conference on Petroleum Phase Behavior and Fouling, New Orleans, LA, March 10-14, 2002. (23) Spiecker, P. M.; Kilpatrick, P. K. Langmuir 2004, 20, 4022–4032. (24) Mukherjee, S.; Kushnick, P. Chemicals used in oil-field operations. In OilField Chemistry: Enhanced Recovery and Production Stimulation; ACS Symposium Series 396; Borchardt, J. K., Yen, T. F., Eds.; American Chemical Society: Washington, DC, 1989; pp 364-374. (25) Krawczyk, M. A.; Wasan, D. T.; Shetty, C. S. Ind. Eng. Chem. Res. 1991, 30, 367–375. (26) Bhardwaj, A.; Hartland, S. J. Dispersion Sci. Technol. 1993, 14, 541–557. (27) Zhang, L. Y.; Xu, Z. H.; Masliyah, J. H. Langmuir 2003, 19, 9730–9741. (28) Ese, M. H.; Galet, L.; Clausse, D.; Sjoblom, J. J. Colloid Interface Sci. 1999, 220, 293–301. (29) Daniel-David, D.; Le Follotec, A.; Pezron, I.; Dalmazzone, C.; Noik, C.; Barre, L.; Komunjer, L. Oil Gas Sci. Technol. 2008, 63, 165–173. (30) Pena, A. A.; Hirasaki, G. J.; Miller, C. A. Ind. Eng. Chem. Res. 2005, 44, 1139–1149. (31) Zhang, Z. Q.; Xu, G. Y.; Fang, W.; Dong, S. L.; Chen, Y. J. J. Colloid Interface Sci. 2005, 282, 1–4.
Langmuir 2010, 26(5), 3050–3057
Article
Figure 1. Structure of EC with a degree of substitution of 2.
triamine-based EO/PO copolymers is more effective in breaking up W/O emulsions than with linear chain structure polymers.31 Effective demulsification of W/O emulsions remains a challenge despite the large number of demulsifiers that have been developed. The commercially used demulsifiers are formulated based mainly on EO/PO copolymers.8 Recently, we reported a novel biodegradable polymer, ethylcellulose (EC) (see structure in Figure 1) for effective demulsification of water-in-diluted bitumen emulsions. In this study, we explore demulsification mechanisms of water-in-diluted bitumen emulsions by EC. The demulsification process with EC is investigated by the in situ micropipet technique at the level of individual micro water droplets in diluted bitumen.32-34 The interfacial films of bitumen at the naphthawater interface in the presence and absence of EC are prepared by a Langmuir interfacial trough. The morphology of the transferred LB films is examined by atomic force microscopy.
Experimental Section Materials. Vacuum distillation feed bitumen and industrialgrade naphtha were provided by Syncrude Canada Ltd. Heptaneinsoluble asphaltene content in bitumen was determined to be ∼11 wt%. Heavy naphtha was supplied by Champion Technologies Inc. HPLC-grade toluene and acetone were purchased from Fisher Scientific. Ethylcellulose (EC) with 48% ethoxyl was purchased from Sigma-Aldrich and used as received. The molecular weight of the polymer was determined by intrinsic viscosity measurement to be 46 kDa.35 EC was dissolved in heavy naphtha and stirred for 24 h prior to its use. The water used to prepare water-in-diluted bitumen emulsions was plant recycle process water from Syncrude Canada Ltd. The process water at pH 8.9, contained 25 ppm of Mg2þ, 41 ppm of Ca2þ, 79 ppm of SO42-, 527 ppm of Naþ, 22 ppm of Kþ, 407 ppm of Cl-, and 793 ppm of HCO3-. Milli-Q water was used in the interfacial tension measurement and Langmuir-Blodgett interfacial film preparation experiments. Interfacial Tension Measurement. The interfacial tension of naphtha-diluted bitumen-water interface in the presence and absence of ethylcellulose was measured using a Processor tensiometer K12 (Kr€ uss, Hamburg, Germany) equipped with a Pt-Ir ring. Milli-Q water was used as aqueous phase. All the measurements were performed at 20.0 ( 0.5 °C. Micropipets Visualization of Water Droplet Interactions. Water-in-diluted bitumen emulsions were prepared with plant process water and naphtha-diluted bitumen at room temperature. The naphtha/bitumen mass ratio in the diluted bitumen was fixed at 0.65 as encountered at industrial practices. After shaking in a mechanical shaker for 4 h at 200 cycles/min, the diluted bitumen was mixed with process water using a homogenizer (PowerGen, 125 W), running at 8000 rpm for 5 s to obtain water droplets with a diameter of 5-30 μm, which is within the size diameter range encountered in industrial practices and convenient for investigation (32) Yeung, A.; Dabros, T.; Masliyah, J.; Czarnecki, J. Colloids Surf., A 2000, 174, 169–181. (33) Tsamantakis, C.; Masliyah, J.; Yeung, A.; Gentzis, T. J. Colloid Interface Sci. 2005, 284, 176–183. (34) Yeung, A.; Dabros, T.; Czarnecki, J.; Masliyah, J. Proc. R. Soc. London, Ser. A 1999, 455, 3709–3723. (35) Feng, X.; Xu, Z.; Masliyah, J. Energy Fuels 2009, 23, 451–456.
DOI: 10.1021/la9029563
3051
Article using micropipet tests. With longer or stronger homogenizing, the size of water droplets would be too small for micropipet experiments. The resultant emulsions contained 5 wt% water. Various amounts of 0.5 wt% EC in heavy naphtha solution were then added to the emulsions under agitation in a water bath sonicator (Fisher Scientific, model no. F56) at 60 °C for 10 s. The emulsions were further diluted with naphtha to give a final bitumen concentration of 0.1 wt% in the emulsions to facilitate visualization. A 30-min equilibration period was allowed before the emulsions were used for investigation of water droplet interactions by the in situ micropipet technique.32,33 Micropipets with a tip of 5-10 μm inner diameter were prepared following the procedures reported elsewhere.32 In micropipet experiments, ∼50 μL emulsion was placed in a sample cell which was assembled from microscope coverslips. An appropriate amount of Milli-Q water was then placed in the sample cell to prevent the evaporation of components in the emulsion. Two suction micropipets were mounted on micromanipulators (Narishige, Tokyo) to enable the pipettes to be positioned at the micrometer resolution. To facilitate the capture of water droplets in emulsions without breaking the droplets, the surface of the micropipets was rendered hydrophobic by soaking in 5 wt% dichlorodimethylsilane-in-cyclohexane solutions. The hydrophobized micropipets were extended into the sample cell from opposite sides. A low level of vacuum at the other ends of the micropipets was applied through syringes to pick up and hold emulsified water droplets. The pressure inside the micropipet was measured by a commercial pressure transducer (Omega Engineering, Stamford, CT). The real time visualization of the micropipet experiments was achieved using a video system (CCD camera, video cassette recorder, and TV monitor) connected to an optical microscope.36 Langmuir-Blodgett Film Preparation. Langmuir-Blodgett (LB) films were prepared using a Langmuir interfacial trough (KSV instruments, Finland). The trough and barriers were rinsed with HPLC-grade toluene prior to each measurement. The interfacial pressure was measured by a Wilhelmy plate (filter paper Whatman 1 CHR) attached to a microbalance. Heavy naphtha, instead of industrial-grade naphtha, was used as topphase in this set of experiments to avoid potential artifacts from the impurities of the industrial-grade naphtha in true interfacial LB films. The naphtha-water interfacial films were transferred onto hydrophilic silicon wafers using LB technique. The silicon wafers were purchased from Ultrasil, Hayward, CA. They were 0.5 μm in thickness and 10 cm in diameter and were polished on one side. Prior to their use as substrate, they were cut to 1 3 cm2 pieces and cleaned by soaking in a 70/30 by volume of 96% sulfuric acid and 30% hydrogen peroxide at 90 °C for 30 min,37 followed by thorough rinsing with Milli-Q water. The cleaned silicon wafer was stored in Milli-Q water prior to its use. Two protocols were used for bitumen interfacial film preparation. In the first, known as spreading protocol, the lower compartment of the Langmuir interfacial trough was filled with 120 mL of Milli-Q water as subphase. The water subphase was considered clean if the pressure sensor reading is below 0.1 mN/m upon barriers compression to the closest position possible. After attaining appropriate level of cleanliness, a clean and dry silicon wafer was immersed in the water subphase. A mixture of 20 μL of 1 mg/ mL solid-free naphtha-diluted bitumen and 20 μL of 1 mg/mL ethylcellulose-in-naphtha solution was then deposited dropwise and evenly on the water surface using a Hamilton microsyringe. Following a 20-min evaporation of naphtha to allow the formation of a uniform surface film on the subphase, 100 mL of naphtha was slowly added as top phase using a glass funnel. The compression of the interfacial film started after 30-min equilibration. For comparison, a separate test with 20 μL of 1 mg/mL solid-free (36) Moran, K.; Yeung, A.; Masliyah, J. Langmuir 1999, 15, 8497–8504. (37) Pintchovski, F.; Price, J. B.; Tobin, P. J.; Peavey, J.; Kobold, K. J. Electrochem. Soc. 1979, 126, 1428–1430.
3052 DOI: 10.1021/la9029563
Feng et al. naphtha-diluted bitumen was conducted using the exact procedures described above. In the second protocol, known as diffusion protocol, 20 μL of 1 mg/mL bitumen instead of bitumen/ethylcellulose mixture was deposited on the water surface after a clean and dry silicon wafer was immersed in the water subphase. A 20-min period of evaporation of naphtha from the spread bitumen solution was allowed before 100 mL of naphtha was added as the top phase. Immediately followed by top phase addition, 20 μL of 1 mg/mL EC was injected evenly into naphtha top phase. In this case, EC was allowed to diffuse through the top phase to the naphtha-water interface for 2 h prior to compression of interfacial films. The interfacial film was compressed at 10 mm/ min to 2 mN/m interfacial pressure and transferred onto the silicon wafer under constant interfacial pressure while the substrate was pulled upward through the interface at a 5 mm/min pulling rate. Atomic Force Microscopy Imaging. Images of LB films were obtained using a Multimode atomic force microscope (AFM) with a nanoscope IIIa controller (Veeco, Santa Barbara, CA) operating under tapping mode in air. AFM imaging was carried out at room temperature (20 ( 0.5 °C) using a multimode scanning probe microscope head and a J-scanner. A silicon tip (RTESP, Veeco) with a resonance frequency of 260-320 kHz was used for imaging at the scan rate of 1 Hz. Images were obtained at several locations for each sample. At least two individual samples prepared under the same deposition condition were imaged. Interaction Force Measurement with AFM. The interaction forces between two immobilized bitumen surfaces on silica in naphtha with and without EC addition were measured using an Agilent 5500 molecular imaging AFM (Agilent Technologies, Santa Clara, CA), equipped with a completely sealed liquid cell by a Kalrez O-ring (Compound 4079, DuPont). The procedures for interaction force measurements were described elsewhere.38 Prior to the force measurement, bitumen film was deposited on silicon wafers and spherical silica probes (8 μm in diameter) by the LB spreading method as described earlier. In the preparation of bitumen surface on a silica probe, the probe was first glued onto the tip of a short, wide beam AFM cantilever using an epoxy adhesive (EP2LV, Master Bound, Hackensack, NJ) cured at room temperature for 2 days. The cantilever (NP-OW, triangular-shaped silicon nitride, spring constant 0.58 N/m, Veeco) with the glued silicon probe was sandwiched between two hydrophilic silicon wafers for deposition of bitumen films onto the probe. Before the force measurement, a bitumen-coated silicon wafer was mounted in the liquid cell as substrate. The cantilever with a bitumen-coated silicon probe attached to the tip was mounted on the scanner (N9524AUS08380117) which was placed above the silicon substrate. The liquid cell was filled with naphtha or 130 ppm of EC in naphtha solutions using a glass syringe and Teflon tubing. A 30-min incubation time was allowed prior to force measurement. When the probe was brought to approach or retract away from the bitumen surface on the silicon substrate, the interactions between the two bitumen surfaces caused the cantilever to deflect. The interaction force was determined by multiplying the deflection of the cantilever by the spring constant of the cantilever, and the force profile between the probe and the bitumen-coated silicon substrate was recorded. The measured long-range and adhesion (pull-off) forces were normalized by the radius of the spherical probe. All the measurements were performed at room temperature of 20 ( 0.5 °C. To ensure a representative force profile, force measurements were carried out at several locations on the bitumen substrate for a given bitumen substrate-bitumen probe pair. At least two pairs for each system were tested in the force measurement. (38) Wang, S. Q.; Liu, J. J.; Zhang, L. Y.; Xu, Z. H.; Masliyah, J. Energy Fuels 2009, 23, 862–869.
Langmuir 2010, 26(5), 3050–3057
Feng et al.
Article
Figure 2. Water content at 2.5 cm from the top of emulsion surface as a function of EC dosage. Emulsions having 60 wt% naphtha-diluted bitumen and an initial 5 wt% plant process water or Milli-Q water were treated with EC and settled under gravity at 80 °C for 1 h. Micrographs of emulsions without polymer addition and with 130 ppm EC addition were shown on the right. The emulsions for optical microscope observation were taken at 6.5 cm from the emulsion surface, i.e., from the bottom of the test tubes.
Results and Discussion The demulsification efficiency of EC was evaluated by measuring the water content at 2.5 cm from the top of emulsion surface and the size of water droplets at 6.5 cm, which is the bottom of the emulsion after settling. Water content was determined using a standard Karl Fischer titrator. The size of water droplets was obtained using a Carl Zeiss Axioskop 40 Pol microscope equipped with a video camera which was connected to a computer. Figure 2 shows water content as a function of EC dosage in emulsions, along with two typical micrographs of emulsions in the absence and presence of 130 ppm EC. Addition of EC caused a rapid decrease in water content. At 130 ppm EC addition, for example, the water content reduced to 0.5 wt%, indicating a 90% removal of the water from the emulsion. A corresponding increase in size of water droplets by 5-10 folds was observed with 130 ppm EC addition, demonstrating the effectiveness of EC as a demulsifier for water in naphtha diluted bitumen emulsions by promoting coalescence of water droplets. It is interesting to note a negligible difference in demulsification efficiency for emulsions prepared with Milli-Q water and the plant process water, although the plant process water contained various types and concentrations of cations, anions, and native surfactants. This observation justifies the use of Milli-Q water in other experiments in this study. It should also be noted that EC alone does not stabilize 5 wt% water-in-naphtha emulsions as phase separation of water from naphtha occurs shortly after emulsification. To understand demulsification mechanisms of EC, the interfacial properties of EC was investigated. The affinity of EC for the oil-water interface was demonstrated by measuring the interfacial tension between water and naphtha with and without bitumen as a function of EC concentration in the oil phase. As shown in Figure 3, 60 wt% bitumen in naphtha caused 3 mN/m reduction of the naphtha-water interfacial tension, whereas 2 ppm of EC could decrease the naphtha-water interfacial tension from 29 to about 25 mN/m. These results indicate that EC has a stronger affinity than bitumen to the naphtha-water interface. As a result, EC could partially displace bitumen components at the naphtha-water interface. This can be seen from results of Figure 3. For example, in the presence of 60% bitumen, 10 ppm of EC caused about 15 mN/m reduction of the naphtha-water interfacial tension, indicating partial displacement of interfacial Langmuir 2010, 26(5), 3050–3057
Figure 3. Interfacial tension between naphtha and water in the presence and absence of 60 wt% bitumen as a function of EC concentration at 20 °C.
bitumen components by EC. Increasing dosage of EC resulted in a further decrease of interfacial tension as a result of displacement of more bitumen components adsorbed at the naphtha-water interface by EC. The difference between naphtha-water interfacial tensions with and without presence of 60% bitumen became almost negligible at high dosage of EC. This finding suggests that EC is capable of substituting most of the surface active bitumen components at the naphtha-water interface as long as a sufficient amount of EC is available. Interestingly, the variation of interfacial tension with EC dosage in Figure 3 is quite similar to that of water content in water-in-naphtha-diluted bitumen emulsions with EC dosage, as shown in Figure 2. This similarity suggests that dewatering efficiency of EC is related to the presence of EC at naphtha-water interface as shown by the reduction of the naphtha-diluted bitumen-water interfacial tension with EC addition. Such a correlation between interfacial tension and water content is revealed in Figure 4. It appears that to effectively demulsify water-in-naphtha-diluted bitumen emulsions from 5 to below 0.5 wt% water content, a critical interfacial tension below 22 mN/m is required. Further decrease of water content requires a much greater reduction in the interfacial tension. However, it should not DOI: 10.1021/la9029563
3053
Article
Figure 4. Relationship between water content in naphtha-diluted bitumen with application of EC and 40 wt% naphtha-diluted bitumen (i.e., 60 wt% bitumen)-water interfacial tension.
be confused here that the lower interfacial tension is the reason for demulsification of water in naphtha-diluted bitumen emulsions. The reduction of interfacial tension is just a measure of stronger surface activity of EC and the presence of EC at the interface is responsible for emulsion breaking, as demonstrated earlier that EC alone could not stabilize water-in-naphtha emulsion. The interfacial activity of EC is attributed to the amphiphilic nature of its molecular structure which comprises hydrophilic cellulose backbone and hydrophobic ethyl substituents. On the basis of the monomer unit of EC and Davies formula,39 the hydrophile-lipophile balance (HLB) value of EC was estimated to be around 8, which is within the range of 8-11 required as demulsifiers for W/O emulsions.40 To understand the demulsification process, the interactions between two emulsion drops were visualized with in situ micropipet technique.32,33 Figure 5 shows sequential still images of two micrometer-sized water droplets in naphtha-diluted bitumen with and without EC addition. When two water droplets were brought in contact for a few minutes under a given applied compression force in the naphtha-diluted bitumen without EC addition, no significant change in the shape of water droplets was observed (A1) and the water droplets returned to their original shape without noticeable deformation upon removal of the compression force (A2). This observation suggests that there was no attractive force (coagulation) between the two water droplets in naphthadiluted bitumen, which is mostly likely due to the presence of protecting interfacial films of surface-active components in bitumen at the naphtha-water interface. With 35 ppm EC addition to the emulsion, an attempt was made to separate the two water droplets after bringing them in contact as shown in micrograph B1. In this case, the water droplets adhered to each other and stretched substantially before final separation when they were pulled apart as shown in micrograph B2, indicating the presence of a strong adhesion force between the water droplets and their flocculation by EC without coalescence. With further increasing EC dosage to 130 ppm, the two water droplets flocculated immediately upon contact (C1) and coalesced shortly to one larger droplet (C2). The whole process of flocculation and coalescence occurred within several seconds, which could be accelerated by further increase in polymer dosages. This observation indicates that coalescence occurred only at a higher dosage of EC. As expected, in the absence of natural surfactants from (39) Davies, J. T. Proc. 2nd Int. Congr. Surf. Activity 1957, V1, 426. (40) Taylor, S. E. Chem. Ind. 1992, 770–773.
3054 DOI: 10.1021/la9029563
Feng et al.
bitumen, water droplets in naphtha flocculated (D1) and coalesced (D2) easily in the presence of 35 ppm of EC, accounting for the observed spontaneous phase separation of water in ECcontaining naphtha, further confirming that the adsorption/ presence of EC at naphtha-water interface is unable to stabilize water-in-naphtha emulsions. The observation from in situ micropipet experiments suggests that water droplets in naphtha-diluted bitumen are stabilized by protecting films of surface active components in bitumen at the naphtha-water interface, as schematically shown in Figure 5-A3. When two water droplets were pressed toward each other under external forces, the liquid between them was squeezed out to the bulk, causing film thinning and nonuniform distribution of surface active materials at the interface. This often generates interfacial tension gradient along the interface which is opposite to the outward liquid flow. In the absence of demulsifiers, the intervening liquid thinning process could be terminated by migration of the natural surfactants (self-repairing) along the naphtha-water interface induced by interfacial tension gradient, known as the Gibbs-Marangoni effect,41 leading to stable emulsions. Emulsions could also be stabilized by repulsive steric forces arising from asphaltene brushes at naphtha-water interface.42,43 By adding a small amount of more surface active EC, EC molecules adsorbed to a partial coverage at the naphtha-diluted bitumenwater interface by penetrating the protecting interfacial films. Such adsorption of relatively large molecules leads to bridging flocculation by its loops and tails, as illustrated in Figure 5-B3. However, the water droplets remain protected by the original interfacial films at such low dosage of EC. As a result, very limited coalescence was observed. Further increase in EC dosage leads to partial displacement of original protecting interfacial films with loosely bound EC containing loops and tails for flocculation while creating voids (passages) in the interfacial films for water to connect and hence coalesce, as schematically illustrated in Figure 5-C3. The presence of loosely packed EC at the naphthawater interface allows a reduction of the interface upon coalescence of two water droplets without need to detach any interfacial materials, leading to a fast coalescence. In the water-in-naphtha emulsions without bitumen, water droplets have no protecting layer on the surface. In this case, the presence of EC could not prevent flocculation and coalescence of water droplets, as shown in Figure 5-D3, leading to observed spontaneous phase separation of water from EC-containing naphtha. To understand demulsification mechanisms of EC from a molecular viewpoint, interfacial films of bitumen and EC/bitumen mixtures transferred from the naphtha-water interface by the LB method were imaged using an AFM. Both the spreading and diffusion methods were used to prepare interfacial films. The spreading method allows us to elucidate whether EC could inhibit the formation of continuous bitumen interfacial films, whereas the diffusion method allows us to probe whether EC could break up bitumen interfacial films that have already formed at the interface before EC was introduced. The diffusion method simulates a real dewatering process in which demulsifier molecules diffuse through naphtha phase onto the surface of water droplets dispersed in diluted bitumen. Figure 6 shows a typical AFM image of bitumen interfacial LB films transferred from the naphtha-diluted bitumen-water interface. The bitumen film was characterized by random, (41) Zapryanov, Z.; Malhotra, A. K.; Aderangi, N.; Wasan, D. T. Int. J. Multiphase Flow 1983, 9, 105–129. (42) Wasan, D. T.; Nikolov, A. D. Aust. J. Chem. 2007, 60, 633–637. (43) Wang, S.; Liu, J.; Zhang, L.; Masliyah, J.; Xu, Z. Langmuir 2009, ASAP July 31, 2009.
Langmuir 2010, 26(5), 3050–3057
Feng et al.
Article
Figure 5. Interactions of water droplets visualized by the micropipet technique. Water droplets were present in 0.1 wt% naphtha-diluted bitumen emulsion (A1, A2, B1, B2, C1, and C2) or in naphtha (D1 and D2). (A1) Water droplets were brought into contact with each other in the emulsion without EC addition; (A2) water droplets were detached from each other, generating no significant change in shape and size in the emulsion containing no EC; (B1) water droplets were brought into contact with each other in the emulsion with 35 ppm EC addition; (B2) water droplets were detached from each other after generating significant deformation (stretching) of flocculated droplets in the presence of 35 ppm EC; (C1) water droplets were brought into contact with each other in the emulsion with 130 ppm EC addition; (C2) water droplets coalesced into one large droplet in the emulsion with 130 ppm EC addition; (D1) water droplets were brought into contact with each other in naphtha with 35 ppm EC addition; and (D2) water droplets coalesced into one large droplet in naphtha with 35 ppm EC addition. A3, B3, C3, and D3 are schematic representations of A1, B1, C1, and D1, respectively. A detailed explanation is given in the text.
close-packed discoid nanoaggregates which are similar to those reported for asphaltenes.27,44,45 Figure 7 shows the morphology of LB film formed by spreading mixture of EC and bitumen at the naphtha-water interface. The EC/bitumen mass ratio was 0.15:1 in the mixture. It is clear that the addition of EC to the naphtha-diluted bitumen prevented the formation of continuous, close-packed discoid bitumen interfacial film shown in Figure 6 and resulted in a partially broken bitumen film. With increasing EC/bitumen mass ratio to 1:1, the (44) Chandra, M. S.; Xu, Z. H.; Masliyah, J. H. Energy Fuels 2008, 22, 1784– 1791. (45) Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. Langmuir 2006, 22, 4487–4497.
Langmuir 2010, 26(5), 3050–3057
bitumen interfacial film was mostly broken. As shown in Figure 8, bitumen film was pushed to a few microdomains of similar nanostructures but much greater film thickness, while EC occupied sporadically the majority of the interface. It is such sporadically distributed area of EC that presented opportunity for water droplets to flocculate and subsequently coalesce, resulting in effective demulsification of the emulsions, as shown in Figure 2. To further reveal the action of EC at the interface, the bitumen film was first formed at the naphtha-water interface and then EC was diffused through naphtha top phase to the interface. The AFM image of the LB interfacial film formed by such a diffusion method is shown in Figure 9. Clearly, EC was able to effectively break up the bitumen interfacial film. DOI: 10.1021/la9029563
3055
Article
Figure 6. AFM image of bitumen LB interfacial film transferred from the naphtha-water interface at 2 mN/m interfacial pressure. The film was prepared by spreading 20 μL of 1 mg/mL bitumen at the interface. The scale of the image on the left is 5 μm.
Figure 7. AFM image of the bitumen/EC LB interfacial film transferred from the naphtha-water interface at 2 mN/m interfacial pressure. The film was prepared by spreading a mixture of 20 μL of 1 mg/mL bitumen and 20 μL of 0.15 mg/mL EC at the interface. The scale of the image on the left is 5 μm.
The stabilization of W/O emulsions has been found to depend on the structure of surface-active components of bitumen at the oil-water interface.46,47 For example, association of surfaceactive components of bitumen would build an interfacial film of significant mechanical strength to protect water droplets. The EC addition to break this stable film relies on either penetration of EC into the structure of interfacial protecting films or simply displacement of interfacial film by EC due to its stronger surface activity. Should EC penetration into protecting interfacial film be dominant mechanism, we would anticipate an attractive force between two bitumen-coated surfaces in EC-containing naphtha; otherwise, the displacement of interfacial film would be the main mechanism as EC would not penetrate the immobilized bitumen film and hence would show a minimum effect on interaction forces between immobilized bitumen films with EC addition. To test this hypothesis, the interaction forces between two bitumen (46) Freer, E. M.; Radke, C. J. J. Adhes. 2004, 80, 481–496. (47) Jeribi, M.; Almir-Assad, B.; Langevin, D.; Henaut, I.; Argillier, J. F. J. Colloid Interface Sci. 2002, 256, 268–272.
3056 DOI: 10.1021/la9029563
Feng et al.
Figure 8. AFM image of the bitumen/EC LB interfacial film transferred from the naphtha-water interface at 2 mN/m interfacial pressure. The film was prepared by spreading a mixture of 20 μL of 1 mg/mL bitumen and 20 μL of 1 mg/mL EC at the interface. The scale of the image on the left is 5 μm.
Figure 9. AFM image of bitumen/EC LB interfacial film transferred from the naphtha-water interface at 2 mN/m interfacial pressure. The film was prepared by diffusing 20 μL of 1 mg/mL EC through naphtha top phase to the interface in which a film was already formed by 20 μL of 1 mg/mL bitumen. The scale of the image on the left is 5 μm.
surfaces were measured in naphtha-diluted EC solution. The results in Figure 10 show that as the two surfaces approached each other a repulsive force was observed for bitumen surfaces in naphtha with and without EC addition. Asphaltenes and resins in bitumen are known to be macromolecules with nonpolar aliphatic hydrocarbon branches and polyaromatic rings along with oxygen-, sulfur-, and/or nitrogencontaining polar functional groups. In naphtha solution, high polarity functional groups of these macromolecules are forced to be interact with and immobilized on hydrophilic silicon surface, while their nonpolar branches enjoy protruding into the naphtha solution, forming tails and loops extending from the silicon surfaces. It is these tails and loops that oppose the approach of two surfaces. The results in Figure 10 show a longer range and stronger repulsive force with than without EC addition. The increased range of repulsion indicates that EC extended the formation of tails and loops from the adsorbed bitumen. It appears that EC adsorbed on bitumen surfaces from naphtha solutions, forcing longer loops and tails into naphtha. Upon Langmuir 2010, 26(5), 3050–3057
Feng et al.
Article
EC addition indicates that EC acted as a bridge to strengthen connection of two bitumen surfaces in naphtha solution, which is consistent with the observed bridging of water droplets in naphtha-diluted bitumen in our in situ micropipet experiments. In the practice of demulsifying water-in-naphtha-diluted bitumen emulsions, water droplets were brought together by gravity or agitation. Bridging flocculation caused by EC would promote coalescence of water droplets and finally results in breakup of emulsions.
Figure 10. Normalized interaction forces between two bitumencoated surfaces in naphtha without (A) and with (B) 130 ppm EC addition as a function of the separation distance, measured at 20 °C: open symbols for approaching and solids symbols for retracting.
Conclusions This study clearly shows that effective demulsification of waterin-diluted bitumen emulsions by EC is a combination of the following mechanisms: (1) EC displaces or breaks up the interfacial film formed by surface-active components in bitumen at the naphtha-diluted bitumen-water interface due to more surface active nature of EC, and (2) EC flocculates water droplets through bridging flocculation leading to their coalescence. In summary, EC breaks up water-in-diluted bitumen emulsions through flocculation-assisted coalescence of water droplets as a result of displacement and disruption of the interfacial films.
retraction, a slight increase in adhesion force from 0.15 mN/m for the system without EC addition to 0.20 mN/m for the system with 130 ppm EC addition was observed. The ultimate separation distance for pulling off two surfaces was extended from 20 to 40 nm by EC addition. The adhesion in the absence of EC is presumably due to the entanglement of loops and tails from the opposite surfaces upon compression. The increased adhesion with
Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council (NSERC) Industrial Research Chair Program in Oil Sands Engineering. We thank Syncrude Canada Ltd. for bitumen samples and access to micropipet equipment. Champion Technologies Inc. is acknowledged for providing the heavy aromatic sample.
Langmuir 2010, 26(5), 3050–3057
DOI: 10.1021/la9029563
3057
